1. Field of the Invention
The present invention relates to an optical scanner using micro mirrors based on microelectromechanical system (MEMS) techniques, a laser image projector adopting the optical scanner, and a method of driving the laser image projector, and more particularly, to a multipurpose micro optical scanner, a laser image projector using the optical scanner, which has no limitations in high-speed driving associated with appropriate laser beam scanning onto a screen, and a method for driving the laser image projector.
2. Description of the Related Art
For laser image projectors, a laser beam is scanned in both horizontal and vertical directions to form an image on a screen. For a general NTSC (National Standard System Committee) image signal, a laser beam is scanned horizontally at 15.75 kHz and vertically at 60 Hz. A motion picture consists of 30 image frames per second, and each still image consists of 525 horizontal scan lines (see FIG. 9B). A vertical scanning unit scans once the screen from the top to the bottom while a horizontal scanning unit scans 525 scan lines onto the screen. For the horizontal scanning unit, after horizontal scanning of a single line from the left to the right, there is a need for quickly returning to the left scanning starting point 5-10 times faster than the previous horizontal left-to-right scanning rate so as to prevent light loss.
FIG. 1 shows the structure of an optical system of a conventional laser image projector. A light source 100 is a white-light laser emitting white light. Semiconductor lasers of red (R), green (G), and blue (B) colors, or a wavelength-convertible solid laser may be used as the light source 100. A beam separator 250 separates the white light into R, G, and B monochromic beams. The beam separator 250 includes two dichroic mirrors 670a and 680a, and a high-reflecting mirror 690a. The dichroic mirrors 670a and 680a separate the white light passed through a lens system 220 and an optical path changing high-reflecting mirror 210 into R, G, B beams, and the high-reflecting mirror 690a changes the optical path of the monochromic beam passed through the dichroic mirror 680a. The separated R, G, and B monochromic beams are focused by focusing lenses 640a, 650a, and 660a, are incident on acousto-optic modulators (AOMs) 610, 620, and 630, respectively, and are modulated based upon an image signal. Collimating lenses 640b, 650b, and 660b for collimating the modulated laser beams back into the same parallel beams as those before entering the focusing lenses 640a, 650a, and 660a are disposed next to the AOMs 610, 620, and 630. The R, G, and B beams modulated based upon the image signal are combined into a single combined beam by a beam combinder 650. The beam combinder 650 includes two dichroic mirrors 670b and 680b, and a high-reflecting mirror 690b. The combined beam is incident on a polygonal mirror 800 at an appropriate angle by high-reflecting mirrors 710 and 720. As the combined beam is incident on the polygonal mirror 800 serving as a horizontal scanning unit, the combined beam is horizontally scanned. A horizontal scanning beam passes through relay lenses 310 and 320, which are disposed between the polygonal mirror 800 and a galvanometer 700, and is focused on a mirror side of the galvanometer 700. A laser beam spot focused on the galvanometer 700 is vertically scanned. An image scanned by the polygonal mirror 800 and the galvanometer 700 is projected onto a screen 900 by a reflecting mirror 850 which is disposed above the galvanometer 700 facing the screen 900.
For the conventional laser image projector having the configuration above, the rotating polygonal mirror 800 is used as a horizontal scanning unit. The rotating polygonal mirror 800 is advantageous in that there is no need for quick returning to the initial scanning point described above. However, the polygonal mirror 800 is mechanically rotated, so that there are limitations in increasing the scanning rate and reducing the size. Thus, a small laser television (TV) cannot be implemented with the polygonal mirror 800. For this reason, a micro optical scanner having a structure of MEMS-technique based microactuator has been suggested as a horizontal scanning unit for a small laser TV. However, unlike a mechanical rotational driving method, for a general galvanometer driving method, a return (or flyback) period 5-10 times shorter than a single horizontal line scanning period is required. However, it is very difficult to manufacture a micro optical scanner which satisfies the need for such quick returning.
U.S. Pat. No. 5,025,346 discloses a micro actuator using the electrostatic effect by comb electrodes. This micro actuator includes movable comb electrodes formed on a movable structure, and stationary comb electrodes formed on a stationary structure, wherein the movable and stationary comb electrodes are, alternately arranged. The movable structure is suspended by neighboring supports. This suspension structure oscillates at a horizontal resonant frequency.
For an x-y axial driving, i.e., along two or more axises, more electrodes are required for a driving unit. For example, the driving unit includes at least three electrodes for an one-axial and unidirectional driving, and at least five electrodes for an one-axial and bidirectional driving. U.S. Pat. No. 5,536,988 discloses a multiaxial driving micro actuator as a driving unit having a plurality of electrodes, which is formed in selective areas of a silicon substrate by a thermal-oxidation insulating method.
The conventional micro actuator includes parallel driving comb electrodes formed along the edge of a movable stage or structure, and parallel stationary comb electrodes fixed on a stationary stage. The stationary comb electrodes and the driving comb electrodes are alternately arranged facing each other.
The conventional micro actuators having the configurations have comb electrodes around the edge of the stages, so that the size of the entire microactuating system is enlarged with respect to the stages or movable structure, thereby limiting the applications thereof.
It is a first object of the present invention to provide a miniature optical scanner in which the structure of comb electrodes is efficiently designed.
It is a second object of the present invention to provide a multipurpose optical scanner capable of linear or 2-dimensional scanning.
It is a third object of the present invention to provide a laser image projector adopting the optical scanner having a micro mirror and a method of driving the laser image projector, in which the scanning direction of a horizontal scan line is alternately changed to eliminate a redundant flyback period, so that correct image reproduction can be achieved at a relatively low driving speed.
To achieve the first and second objects of the present invention, there is provided an optical scanner comprising: a base substrate; a plurality of parallel stationary comb electrodes arranged on the base substrate extending upwards at right angle; a stage having a mirror side at its top side, being separated a predetermined distance above the base substrate; a plurality of parallel driving comb electrodes arranged on the bottom of the stage extending at right angle interdigitated with the stationary comb electrodes; torsion bars formed at both side edges of the stage with a predetermined length to support such that the stage pivots; and supports for supporting the torsion bars such that the stage is suspended above the base substrate.
In another embodiment, there is provided an optical scanner comprising: a first optical scanner having a reflecting side from which an incident beam is reflected in a first direction within the range of a predetermined angle; and a second optical scanner for scanning a laser beam scanned by the first optical scanner in the first direction, in a second direction perpendicular to the first direction, wherein each of the first and second optical scanners comprises: a base substrate; a plurality of parallel stationary comb electrodes arranged on the base substrate extending upwards at right angle; a stage having a mirror side at its top side, being separated a predetermined distance above the base substrate; a plurality of parallel driving comb electrodes arranged on the bottom of the stage extending at right angle interdigitated with the stationary comb electrodes; torsion bars formed at both side edges of the stage with a predetermined length to support such that the stage pivots; and supports for supporting the torsion bars such that the stage is suspended above the base substrate.
According to an aspect of the third object of the present invention, there is provided a laser image projector comprising: a light source for emitting a white light beam; a beam separating unit for separating the white light beam into three main monochromic beams; an acousto-optic modulating unit for modulating the three main monochromic beams based on color signals; a beam combining unit for combing the monochromic beams modulated by the acousto-optic modulating unit into a single combined beam; and a laser scanning unit for scanning the single beam combined with the monochromic beams to form an image, wherein the laser scanning unit comprises a horizontal scanning mirror for horizontally scanning the single combined beam, and a vertical scanning mirror for vertically scanning a beam incident from the horizontal scanning mirror.
In the laser image projector, it is preferable that the laser scanning unit comprises a first optical scanner having the horizontal scanning mirror and a second optical scanner having the vertical scanning mirror, wherein each of the first and second optical scanners comprises: a base substrate; a plurality of parallel stationary comb electrodes arranged on the base substrate extending upwards at right angle; a stage having a mirror side at its top side, being separated a predetermined distance above the base substrate; a plurality of parallel driving comb electrodes arranged on the bottom of the stage extending at right angle interdigitated with the stationary comb electrodes; torsion bars formed at both side edges of the stage with a predetermined length to support such that the stage pivots; and supports for supporting the torsion bars such that the stage is suspended above the base substrate.
It is preferable that the horizontal scanning mirror horizontally scans the single beam combined with the monochromic beams alternately from the left to the right and from the right to the left. It is preferable that the laser scanning unit further comprises a memory for temporarily storing an image information horizontally scanned from the right to the left.
According to another aspect of the third object of the present invention, there is provided a method of driving a laser image projector including: a light source for emitting a white light beam; a beam separating unit for separating the white light beam into three main monochromic beams; an acousto-optic modulating unit for modulating the three main monochromic beams based on color signals; a beam combining unit for combing the monochromic beams modulated by the acousto-optic modulating unit into a single combined beam; and a laser scanning unit for scanning the single beam combined with the monochromic beams to form an image, the method comprising alternately horizontally scanning the single beam combined with the modulated monochromic beams from the left to the right and from the right to the left.
It is preferable that an image signal scanned in opposite directions to an input image signal in the alternate horizontal scanning from the left to the right and from the right to the left is temporarily stored in a memory and is output in the reverse order such that a correct image can be displayed.